In conventional magnetic recording, thermal instabilities of the stored magnetization in the recording media can cause loss of recorded data. To avoid this, media with high magneto-crystalline anisotropy (Ku) are required. However, increasing Ku also increases the coercivity of the media, which can exceed the write field capability of the write head. Since it is known that the coercivity of the magnetic material of the recording layer is temperature dependent, one proposed solution to the thermal stability problem is thermally-assisted recording (TAR), also called heat-assisted magnetic recording (HAMR), wherein high-Ku magnetic recording material is heated locally during writing to lower the coercivity enough for writing to occur, but where the coercivity/anisotropy is high enough for thermal stability of the recorded bits at the ambient temperature of the disk drive (i.e., the normal operating or “room” temperature of approximately 15-30° C.). In some proposed TAR systems, the magnetic recording material is heated to near or above its Curie temperature. The recorded data is then read back at ambient temperature by a conventional magnetoresistive read head. TAR disk drives have been proposed for both conventional continuous media, wherein the magnetic recording material is a continuous layer on the disk, and for bit-patterned media (BPM), wherein the magnetic recording material is patterned into discrete data islands or “bits”.
One type of proposed TAR disk drive uses a “small-area” heater to direct heat to just the area of the data track where data is to be written by the write head. The most common type of small-area TAR disk drive uses a laser source and an optical waveguide with a near-field transducer (NFT). A “near-field” transducer refers to “near-field optics”, wherein the passage of light is through an element with subwavelength features and the light is coupled to a second element, such as a substrate like a magnetic recording medium, located a subwavelength distance from the first element. The NFT is typically located at the air-bearing surface (ABS) of the air-bearing slider that also supports the read/write head and rides or “flies” above the disk surface.
One type of proposed high-Ku TAR media with perpendicular magnetic anisotropy is an alloy of FePt or CoPt alloy chemically-ordered in the L10 phase. The chemically-ordered FePt alloy, in its bulk form, is known as a face-centered tetragonal (FCT) L10-ordered phase material (also called a CuAu material). The c-axis of the L10 phase is the easy axis of magnetization and is oriented perpendicular to the disk substrate. The FePt and CoPt alloys require deposition at high temperature or subsequent high-temperature annealing to achieve the desired chemical ordering to the L10 phase. An insulating crystalline MgO layer is typically located below the FePt layer to enhance the growth of the FePt material and to confine the heat from the NFT to the FePt.
However, a problem associated with a TAR disk is optimization of the amount of heat to the FePt recording layer. If the thermal conductivity of the MgO insulating layer is too high the heat from the NFT will be distributed too rapidly, which will require more laser power to heat the FePt material. If the thermal conductivity of the MgO insulating layer is too low the heat from the NFT will be confined to the recording layer and will spread laterally through the recording layer. This is undesirable because the lateral spreading of the heat may cause recorded data in adjacent data tracks to be overwritten. Thus there is a trade-off in the design of a TAR disk to optimize the properties of the insulating layer and optional heat sink layer to both minimize the amount of heat required and to prevent lateral spreading of the heat through the recording layer.
What is needed is an improved TAR disk that allows for control and optimization of heat to the recording layer.